Income Taxes - Income Tax Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of income taxes computed at the United States federal statutory tax rates
Tax at U.S. statutory rate									$ 39,616	$ 55,955	$ 47,522
Increase (decrease) in taxes resulting from:
Tax-exempt interest income									(1,433)	(3,595)	(3,467)
BOLI income									(975)	(1,524)	(1,622)
Investment tax credits									(1,863)	(1,591)	(1,390)
Amortization of investment in low-income housing tax credits									1,592	1,873	1,742
State income tax expense, net of federal benefit									4,502	1,626	1,747
Revaluation of net deferred tax assets as a result of the Tax Cuts and Jobs Act									0	14,486	0
Other items, net									288	451	315
Total income tax expense	$ 13,098	$ 8,532	$ 10,424	$ 9,673	$ 30,234	$ 14,199	$ 11,993	$ 11,255	$ 41,727	$ 67,681	$ 44,847